The present invention relates to an information recording disc for recording and reproducing information, in more detail, to an information recording disc (hereinunder referred to as a "disc") which is clamped to the turntable of a recording and reproducing apparatus by a magnetic attracting member and, more particularly, to the structure of a recording medium of an optical disc (hereinunder referred to as an "optical disc").
Conventionally, when an optical disc is mounted on a spindle of a recording and/or reproducing apparatus, a disc consisting of two substrates 1 and 1' are supported by the receiving surface of the spindle 2 and are pressed against the spindle 2 from above so as to be fixed by a fixing mechanism 3 (what is called mechanical clamp). All of the compact discs and optical video discs now on the market are of this type. The fixing mechanism 3, however, offers a problem in the attempt to reduce the apparatus in size and weight, because it is necessary to increase the thickness of the apparatus by the thickness of the fixing mechanism 3 plus the distance of travel of the fixing mechanism 3 which is required for moving it upward from the position shown in FIG. 1 when mounting and removing the disc. The fundamental structure of a means for solving this problem is shown in FIG. 2. An example of the means is described in detail in Japanese Laying Open of Utility Model application No. 138030/1984. This is a means for adhering a metal piece or a magnetic material 5 to the outside of the central portion of the disc consisting of the substrates 1 and 1', and attracting the disc by virtue of the attractive force of a magnetic material 4 provided on the spindle 2 (what is called magnetic clamp). As compared with the apparatus shown in FIG. 1, it dispenses with the need for the fixing mechanism 3 and the apparatus is reduced in thickness by that degree. However, when the means shown in FIG. 2 is put to practical use, there is a serious problem of the generation of retardation due to a change in the ambient temperature of the disc. The retardation is caused as a result of the generation of strain stress due to a difference in thermal expansion coefficient between the substrates 1, 1' and the magnetic attracting member adhered thereto, for example, the metal piece or magnetic material 5; it is what is called a photoelastic effect. The retardation disturbs the polarization of recording and reproducing light, thereby introducing deterioration of the recording and reproducing properties. From this reason the allowed value of retardation is considered to be 70 nm with respect to a commercial compact disc (CD) and an optical video disc, 40 nm with respect to what is called a DRAW (Direct Read After Write) type optical disc which is used for image filing and the like, and 10 nm with respect to a magneto-optic disc which detects a slight rotational angle (.about.0.35.degree.) of polarization. Discs having the structure shown in FIG. 2 were made such as not to exceed the allowed value and the retardations generated when the temperature was varied 40.degree. C. were measured. The results are given in FIG. 3. The radius of the discs was 65 mm and the radius of the recording and reproducing region was 30 to 60 mm. The plastic material of the substrate was a polycarbonate resin. The curve a shows the retardation of a commercial plastic magnetic member A of 0.8 mm thick, and b that of a commercial plastic magnetic member B of 0.8 mm thick, the inner and outer diameter of both magnetic members are .phi.15 and .phi.33 mm, respectively. Both magnetic members A and B are composed of a plastic with magnetic materials dispersed therein, and have a thermal expansion coefficient closer to that of plastic than an ordinary metal piece or a magnetic material alone. The curve c shows the retardation of a small-diameter iron ring of 0.1 mm thick. The inner and outer diameters of the iron ring are .phi.15 and .phi.20 mm, respectively. As to the physical characteristic constants in relation to the generation of retardation, the photoelasticity of the polycarbonate resin is 5.5.times. 10.sup.-4 mm.sup.2 /kg, the thermal expansion coefficient thereof is 6.8.times.10.sup.-5 /deg, and the Young's modulus thereof is 240 kg f/mm.sup.2, the thermal expansion coefficients of the plastic magnetic members A and B are 2.46.times.10.sup.-5 /deg and 3.0.times.10.sup.-5 /deg, respectively, and the Young's moduli thereof are 2,100 kg f/mm.sup.2 and 1,170 kg f/mm.sup.2, respectively. The thermal expansion coefficient of the iron ring is 1.2.times.10.sup.-5 /deg and the Young's modulus thereof is 20,000 kg f/mm.sup.2.
Thus, even a plastic magnet generates a retardation of several hundred nm, and cannot therefore be used for a CD or optical disc which has the largest allowed value, much less a metal piece or a strong magnetic material having a smaller thermal expansion coefficient than a plastic magnet. This is approximately the same with the case in which the substrate material is replaced by an epoxy resin, which have approximately the same physical characteristic constants. If the substrate material is PMMA (poly methyl methacrylate), since the photoelastic constant of the PMMA is about 1/10 that of the polycarbonate resin, it is a little better but is far from being utilizable for a magneto-optic disc. FIG. 4 shows the calculated value of the retardation generated when a metal piece which has a thermal expansion coefficient different from that of the epoxy resin by as small as 0.3.times.10.sup.-5 /deg is adhered to the disc as the metal piece 5 shown in FIG. 2. The retardation in this case also exceeds the allowed value (&lt;10 nm) of a magneto-optic disc. Thus, it has been found that adhesion of a metal piece or a magnetic material to a substrate is not practical at all.